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Title: Dynamics from powder diffraction
Author: Goodwin, A. L.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2006
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This dissertation explores the extent to which dynamical information is preserved in neutron powder diffraction data. Total scattering data collected on the GEM instrument in ISIS have been used as input for the reverse Monte Carlo (RMC) method to generate large ensembles of independent atomistic configurations consistent with the data. A method of analysing these configurations for evidence of correlated atomic motion has been developed. The method is based on a similar technique originally used to analyse molecular dynamics (MD) configurations. It has been extended further in the dissertation to account for lattice symmetry, allowing unambiguous assignment of the normal modes at each wavevector. The theory has also been used to show that phonons are determined uniquely by first- and second-order displacement correlation functions alone, and should consequently be fixed by diffraction data. The impact of various restraints and constraints in the RMC method on the dynamical information stored within configurations has been systematically studied and used to develop a new constraint (the DW constraint) that helps prevent the incorporation of damaged regions into RMC configurations without affecting the form of the phonon dispersion curves obtained. The phonons-from-diffraction approach, implemented in the program PHONON, has been applied to MgO and SrTiO3. Calculated phonon dispersion curves resemble those obtained independently in spectroscopic experiments in the low-frequency region; however, the higher-frequency modes were generally not well preserved. Temperature-dependent mode softening of the R25 and Γ15 modes in SrTiO3 could be observed, and the mode displacement vectors used to characterise the atomic translations involved in the 105 K displacive phase transition.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available